Lip of a female patient after the removal of a mucocele with cryo tweezers. A mucocele, or mucus retention cyst, is a swollen sac filled with mucus secreted by its inner lining. They can occur on mucous membranes anywhere in the body. They are generally harmless unless they block or obstruct important areas, but are usually removed surgically to prevent any later complications. For an image of this mucocele being removed see C059/9139.